1. Field of the Invention
The present invention relates to an electronic endoscopic apparatus provided with an isolation circuit which is driven by a differential type drive means in a signal processing system.
2. Description of Prior Art
Various electronic endoscopic apparatuses provided with an image pickup means such as an electronic endoscope in which a solid-state image pickup device such as a charge coupled device (CCD) serving as an image pickup means is provided at the end of an insertion portion and an endoscope in which a camera is provided on an ocular portion of a fiberscope have been recently proposed.
The image pickup method of the image pickup device used in these electronic endoscopic apparatuses fall roughly into two types depending upon the exposure and read methods of the solid-state image pickup devices. The two types include a type containing a color filter which has a color mosaic filter in front of a solid-state image pickup device so as to simultaneously obtain three color signals R, G, and B and a sequential surface type using a black-and-white solid-state image pickup device in which, for example, sequential surface light R, G and B is projected from an illuminating light source and applied to a target so that the three color signals R, G and B are obtained.
The outline of the sequential surface type of the electronic endoscopic apparatus of the prior art is described below with reference to FIGS. 1 and 2.
As shown in FIG. 1, an electronic endoscopic apparatus 1 comprises an electronic endoscope 2, a signal processing apparatus 5 which contains a light source unit 3 for supplying illuminating light to the electronic endoscope 2 and a signal processing circuit unit 4 for processing the image signals obtained by the electronic endoscope 2, and a monitor 6 which is connected to the signal processing apparatus 5.
The electronic endoscope 2 is provided with a long thin, flexible, insertion portion 7 to whose rear end an operational portion 8 with a large diameter is connected. A flexible universal cord 9 is extended sideward from the operational portion 8, and a connector 11 which can be connected to the signal processing apparatus 5 is provided at the end of the universal cord 9.
A hard end portion 12 and a flexible portion 13, which can be curved and which is placed adjacent to the end portion 12 on the rear side thereof, are provided on the end side of the insertion portion 7. A flex control knob 14 is provided on the operational portion 8 so that the flexible portion 13 can be curved in the longitudinal and horizontal directions by rotating the flex control knob 14. An insertion hole 15 communicating with the treatment tool channel provided in the insertion portion 7 is also provided in the operational portion 8.
An objective lens 16 and a solid-state image pickup device 17 are disposed in the end portion of the insertion portion 7, as shown in FIG. 2. A light guide 18 for transmitting illuminating light is also passed through the insertion portion 7.
The light source unit 3 is provided with a white light source 22 such as a xenon lamp. The white light projected from the white light source 22 is condensed by a lens 23, changed into sequential rays of light of R, G and B type by a rotating filter 24 which rotates in synchrony with the frame frequency of a video signal (29.97 Hz in the NTSC mode), and applied to a subject such as an organ to be observed in a human body through the light guide 18 and a distribution lens 19. The rotation of a motor 25 for rotating the rotating filter 24 is controlled by a rotation servo circuit 27 so as to synchronize with the frame frequency of a video signal.
The light reflected from the subject 21 is caused to form an image on the image pickup surface of the solid-state image pickup device 17 through the objective lens 17. The formed image is subjected to photoelectric conversion using a read clock signal from a driver 28 to output sequential signals R, G and B. The above-mentioned clock signal and the reference signal of the rotation servo circuit 27 are supplied from a synchronous signal generator 29 which is a single reference signal source. Therefore, all signals (actions) are phase-synchronized.
The sequential signals R, G and B output from the solid-state image pickup device 17 are amplified by a preamplifier 31 of the signal processing unit 4, passed though an isolation amplifier 32 for protecting a patient from any electric shock, and sent to a reset noise eliminating circuit 33 in which any reset noise is removed.
After unnecessary components have been eliminated by a low-pass filter 34, the sequential signals are subjected to vertical outline compensation by a vertical outline compensating circuit 35 and then to .gamma.-compensation by a .gamma.-compensating circuit 36. The output signals from the .gamma.-compensating circuit 36 are converted into digital signals by an A/D converter 37, and the signals read by the illuminating rays of light types R, G, B are respectively stored in one frame memory of frame memories 38R, 38G and 38B which correspond to the sequential surface illumination. The aforementioned frame memories 38R, 38G and 38B are simultaneously read and changed to simultaneous signals which are then converted into analogue signals by D/A converters 39. The conversion rate of the aforementioned A/D converter 37 and the writing of data in and the reading of data out of each of the frame memories 38R, 38G, 38B are controlled by the output signals from a memory control circuit 43.
After the unnecessary components of each of these simultaneous analogue signals R, G and B have been eliminated by low-pass filters 41, the signals are respectively subjected to horizontal outline compensation by horizontal outline compensating circuits 42, then amplified by power amplifiers 44, and output, for example, as primary color signals of types R, G and B with an output impedance of 75 .OMEGA., from output terminals to the monitor 6.
On the other hand, a luminance signal is formed in an R-Y matrix circuit 46 from each of the R, G, B signals which are made simultaneous and subjected to horizontal outline compensation. A color difference signal R-Y is formed from the luminance signal Y and the color signal R in a R-Y matrix circuit 47 and a color difference signal B-Y is formed from the luminance signal Y and the color signal B in a B-Y matrix circuit 48.
These color difference signals R-Y and B-Y are subjected to equilibrium modulation by sub carriers (signals of 3.5779545 Hz with a phase difference of 90 degrees therebetween) in encoders 49 and 50 respectively and subjected to vector composition in an adder 51 to form a chrominance signal C. This chrominance signal C is multiplexed with the luminance signal Y by a mixing power amplifier 52. A composite synchronous signal and a color burst are added to the multiplexed signal to form a composite image signal of the NTSC mode which is then output from an NTSC output terminal.
The transmission of the output signal from the above-described solid-state image pickup device 17 to the signal processing unit 4 has been performed by the method shown in FIG. 3, which is the simplest transmission method.
The output signal from the solid-state image pickup device 17 is passed through a buffer amplifier 55 (in some cases, having an amplification degree) which is disposed near the solid-state image pickup device 17 and is then transmitted to the signal processing unit 4 from the electronic endoscope 2 through a signal transmission cable 56 which is composed of a shielding wire or a coaxial cable. In the signal processing unit 4, the signal is similarily received by a buffer amplifier or a preamplifier 31 and then transmitted to a processing circuit in the next stage.
The endoscopic apparatus has a function as a surgical instrument, for example, a high-frequency electric knife 60 passed through the treatment tool channel 15 is used for ablation or hemostasis of a diseased part. The electric knife 60 comprises a high-frequency power source 61 and a long thin probe 62 which is passed through the treatment tool channel 15, as shown in FIG. 3. The above-described probe 62 comprises a cable portion 63 which is connected to the high-frequency power source 61 and, for example, a loop-shaped electrode 64 for ablation or hemostasis which is connected to the end of the cable 63. The cable portion 63 is covered with a flexible sheath 65.
When ablation or hemostasis is performed using the electric knife 60, the sheath 65 is passed through the treatment tool channel 15 of the electronic endoscope 2. A high-frequency current (generally about 500 KHz) is caused to flow through a human body from a diseased part thereof by the electrode 64 at the end of the cable portion 63.
The electric knife 60 basically effects ablation and hemostasis of a diseased part by using discharge breakdown and Joule heat caused by the high-frequency current. Therefore, in the conventional example shown in FIG. 3, when the electric knife is used, the high-frequency current flowing through the cable portion 63 is induced as noise in the solid-state image pickup device 17, the buffer amplifier 55 and the signal transmission cable 56 of the electronic endoscope 2. The noise affects the image signal processed in the signal processing unit 4, resulting in the display of an image having much noise and significantly impaired qualities.
An example of a measure to deal with this problem is the noise prevention apparatus disclosed in Japanese Patent Laid-Open No. 55923/1985. In this apparatus, as shown in FIG. 4, one ends of signal transmission cables 67, 68 are respectively connected to the differential output terminals of a buffer amplifier 66 in which the output signal from the solid-state image pickup device 17 is input, add the other ends of the two signal transmission cables 67, 68 are connected to an inversion input terminal and a non-inversion input terminal respectively of a differential amplifier 69 in the signal processing unit 4 signal output from the differential amplifier 69 sent to the processing circuit in the next stag.
It is thought that the noise induced in the buffer amplifier 66 is equally superposed on the positive (+) and negative (-) outputs thereof, and that the noise induced in the signal transmission cables 67, 68 is also equivalent. Therefore, the equivalent noise is induced in the two signal transmission paths and these noise components are offset by the differential amplifier 69.
However, in the prior art shown in FIG. 4, since the noise superposed on GND cannot be sufficiently suppressed when the signal is transmitted to a secondary circuit side by an isolation circuit, the noise is also transmitted to the secondary circuit side. In particular, as the ability to eliminate noise is insufficient to the noise caused by the electric knife 60, an image with impaired qualities is still displayed.
FIG. 5 shows the entire configuration which satisfies the safety standards required for medical instruments and defined by IEC (International Electrotechnical Commission).
In other words, an electronic endoscopic apparatus 71 which is inserted into the body of a patient and used for a diagnosis or as a surgical instrument and which satisfies the aforementioned safety standards requires a means for strictly protecting the patient from any electric shock.
This apparatus is fundamentally divided into a primary circuit in which a commercial power source is input, a secondary circuit 74 in which a sheath 73 is grounded, and a diagnostic circuit 75 (an electrical circuit in which a patient serves as part of the circuit) which is isolated from the ground of the secondary circuit 74. The isolation resisting voltage and the leakage current are specified for each of the relationships between these circuits.
A solid-state image pickup device 77 (referred to as SID hereinafter) is built at the end of the electroscope 2, and a plurality of drive signals are applied to the SID 77 from an SID driver 79 in a processing unit 78 on the body side of the apparatus. These drive signals are transmitted by, for example, coaxial cables 81, 81. In addition, the signal output from SID 77 is amplified by an amplifier 82, transmitted through a coaxial cable 83, amplified by an amplifier 84 into the sheath 73, and then input in a driver 85 for driving an isolation circuit 89.
Power is supplied to each of the above-described two drivers 79 and 85 from a diagnostic circuit power source 86. The power source 86 also supplies driving power to the SID 77.
The driver 79, which outputs the drive signals to the SID 77, is supplied a timing signal from an SID timing signal generator 88 through an isolation circuit 87.
The output from the driver 85, in which the output signal from the amplifier 84 is input, is also input in a signal processing circuit 91 through the isolation circuit 89, and the image signal processed is output from a signal output terminal 92.
Power is supplied to each of the SID timing signal generator 88 and the signal processing circuit 91 from a secondary circuit power source 93. Power is supplied to the portion of each of the isolation circuits 87, 89 on the side of the secondary circuit 74 and to the portion thereof on the side of the diagnostic circuit 75 from the secondary circuit power source 93 and the diagnostic circuit power source 86, respectively. Commercial alternating current power is supplied to the diagnostic power source 86 and the secondary circuit power source 93 from secondary coils 95a and 95b, respectively, which are isolated from each other by a power transformer 94.
In the electric knife 60, the signal output from an oscillator 97 is supplied to the ablation electrode 64 through an isolating transformer 98. The high-frequency output signal supplied to the ablation electrode 64 is returned to the transformer 98 through a plate 99 which has a large area and is brought into a patient.
Consequently, in this electronic endoscopic apparatus 71, the electrical circuit in the electroscope 2 which is inserted into the body of a patient is isolated (meaning that it is floated on) from the ground (the sheath 73 is equivalent to ground) having the most stable reference potential. The electrical circuit including the GND (ground) thereof has thus a configuration which is greatly affected by the noise of the electric knife 60, etc.
An example of a configuration of the circumference of the isolation circuit 89 is shown in FIG. 6.
The output from the driver 85 in which a signal a is input is input in an LED which constitutes a photocoupler serving as the isolation circuit 89. The light emission output from the LED is received by a photodiode PD (or a phototransistor) in which the output is converted into an electrical signal. The cathode of the photodiode PD is connected to the secondary circuit power source 93 through a load resistance Rl.
In this case, for example, if the input signal a shown in FIG. 7(a) is mixed with the noise b of the diagnostic circuit GND shown in FIG. 7(b), a signal c at an output terminal of the isolation circuit 89 is an output signal containing the noise, as shown in FIG. 7(c).
In addition, it is generally necessary to supply as drive signals to the SID 77, which is contained in the scope 2 at the end thereof, a plurality of high-frequency pulses such as horizontal and vertical drive signals. In the transmission of such signals, the coaxial cables 81 described above or the shielding cables are generally used as measures against the effect of noise, cross talk and unnecessary radiation.
Such signals also constitute the diagnostic circuit 75, and it is apparent from FIG. 5 that the diagnostic circuit 75 has a configuration easily affected by the noise due to a loop formed for external noise in the shield portions of the coaxial cables 81 and the above-described floating configuration.
On the other hand, U.S. Pat. No. 4,607,621 proposed an apparatus characterized in that a signal cable in a scope is received in a conductive shield which is electrically connected to a chassis of a signal processing apparatus, and the chassis is connected to a high-frequency ground point of a high-frequency power unit (electric knife body).
However, the above-described U.S. Pat. in practice cannot easily satisfy the resisting voltage of the safety standard defined by IEC between the circuit in the scope and the chassis.
There is consequently the possibility that a problem with respect to guarantee for the safety of a patient still remains.